The file you are trying to open "Personal.xls" in a different format than specified file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?

What version of Excel are you running chatman?

This problem often arises when you change from 2003 to 2010 for example. 

This may be not the relevant answer as you have not specified your version of excel.

I think the version of excel you are trying to open may have been created on different excel version. The solution is to save as the file again under your latest version of excel, so that you will not see the message again

Or, you could check your setting in excel options and see which box has been ticked/unticked.

or better, open , accept the warning, then save as personal.xlsb and delete personal.xls.

If you have hard coded references to PERSONAL.XLS in the macros then better is to open, save as personal.xls being sure to also specify file type Excel 2003 (.xls)

"Which box should I be looking for in my Excel settings? "

Cantona1 was probably referring to Office Button > Excel Options > Save > Save Files in this format: you probably have "Excel 97-2003 *.xls" which is Compatibility Mode. Might be time to change that to one of the top 3 Excel 2007 file formats.

Compatibility Mode:

Sysmod is right! I think you should choose the top format (Escel workbook) as it is compatible with latest version

Since you have also mentioned personal.xls. I am sure you have macros somewhere on your PC. You could go to Office Button > Excel Options>Trust Centre >Trust centre settings, and change the setting.

To open, read, scan, extract and even recover *.xls, *,xlt, *.xlsx, xlsm, *.xltm, *.xltx, *.xlam file formats you may use http://www.recoverytoolbox.com/excel.html